Thermal conductivity vacuum instruments are a class of pressure measurement devices that operate by measuring in some way the rate of heat transfer between a heated element and its surroundings. The heat transfer between a heated element and a nearby wall is pressure dependent when the Knudsen number (Kn) is in the range of 0.01&lt;Kn&lt;10. Examples of instruments of this type are the Pirani-type gauges such as the Granville-Phillips Convectron vacuum gauge that measures vacuum pressures by changes in the resistivity of a heated wire, and thermocouple-type gauges such as the Teledyne Brown Engineering-Hastings Instruments thermocouple-type vacuum gauge which measures pressures by monitoring changes in output voltage of a heated thermocouple.
All of these types of commercially available vacuum sensors exhibit one or more of the following problems: attitude sensitivity, highly nonlinear output signal, narrow dynamic range, fragility of the sensing element, bulky sensor and/or limited overpressure capability.
Another type of thermally controlled pressure sensor, formed of etched and vapor-deposited silicon has a heated thin-film resistor on a membrane, an ambient temperature sensing resistor on a silicon base substrate, and a lid over the membrane to form a measuring chamber. Such sensors can measure pressures in the range of 1000 Torr to 1.times.10.sup.-5 Torr by maintaining the resistor on the membrane at a constant temperature so that the output of the thin-film resistor circuit changes with changing pressure. Dependent upon the dimensions of the chamber formed by the lid over the membrane, heat transfer from the heated resistor is generally by conduction from 0.001 Torr to 100 Torr, and by convection from 100 Torr to 760 Torr. One such sensor is the VPS028 offered by Mannesmann Hartmann & Braun Aktiengesellschaft, Frankfort, Germany. Such sensors have the advantages of small size and attitude insensitivity. However, pressure sensitivity in the range of 100 Torr to 760 Torr is low in sensors of this type which have a lid-to-membrane dimension on the order of a few hundred microns or greater. This is a result of the chamber operation in the transition or viscous flow regimes (depending on the pressure). Only in molecular flow is heat conduction directly proportional to pressure.
Another problem heretofore unsolved with respect to accurate measurement of pressure by miniaturized sensors is the difficulty of measuring small values of voltage or power against relatively large background power levels. For example, at 1.times.10.sup.-5 Torr, the power signal level of the membrane resistor is five orders of magnitude less than the background power level. This would require seventeen bits of accurate resolution in an analog-to-digital converter to resolve the small signal. Measurements of power based only upon applied voltage lose the signal entirely in thermal noise generated by ambient temperature variations.
Other types of sensors use a constant voltage or constant current source to measure power by monitoring changes in resistance, and correcting for ambient temperature variations with non-linear compensator circuitry. However, the complexity of such compensation also interferes with accurate measurement at very low pressures. Electrical noise from parasitic capacitance of signal lines also interferes with accurate measurement of low level signals.